Petschek-type time-dependent reconnection (TDR) and quasi-stationary reconnection (QSR) models ar... more Petschek-type time-dependent reconnection (TDR) and quasi-stationary reconnection (QSR) models are considered to understand reconnection outflow structures and the generation of local turbulence in the solar wind. Comparing TDR/QSR model predictions of the outflow structures with actual measurements shows that both models can explain the data equally well. It is demonstrated that the outflows can often generate more or less spatially extended turbulent boundary layers. The structure of a unique extended reconnection outflow is investigated in detail. The analysis of spectral scalings and spectral break locations shows that reconnection can change the local field and plasma conditions which may support different local turbulent dissipation mechanisms at their characteristic wavenumbers.
Proceedings of the International Astronomical Union, 2004
In this paper we compare the intermittence properties of magnetic fluctuations (non-Gaussian shap... more In this paper we compare the intermittence properties of magnetic fluctuations (non-Gaussian shape of probability density functions) observed in the solar wind (ACE) with the simultaneous occurrence of intermittence in the Earth's plasma sheet (GEOTAIL). Intervals with different level of magnetic turbulence are investigated separately.
Three consecutive turbulent magnetic dipolarizations accompanied by auroral brightenings near the... more Three consecutive turbulent magnetic dipolarizations accompanied by auroral brightenings near the equatorward boundary of wide auroral oval were observed with fortuitous spacecraft constellation on September 26, 2005. All were associated with strong near-Earth reconnection pulses (at r ≤ 14Re with Cluster probed the tailward reconnection outflow region) with separatrix mapped to ∼ 64 • CGLat in the ionosphere where a narrow energy-dispersed ion injection was observed. Onset of magnetic reconnection was nearly simultaneous or lead as compared to the turbulent dipolarization and energetic particle injection onsets. The reconnection tailward outflow contained intense turbulence with the properties similar to that in the turbulent dipolarization regions and with intensity correlating with the outflow amplitude. We conclude that the reconnection process and the growth of strong turbulence in the near tail are strongly coupled together, at least in near-Earth reconnection events, and that near-Earth location of the reconnection site may be more frequent phenomenon than typically thought. In that case it assumed to be possible due to enhanced SW flow pressure which kept the magnetic configuration very stretched in the absence of strong energy loading into the magnetosphere; the ground magnetic perturbations ranged between 50 and 300nT in these intense reconnection events.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this p... more The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.
Journal of Geophysical Research-Space Physics, 2013
We study the development of the proton temperature anisotropy T ⊥ /T || in bursty bulk flows (BBF... more We study the development of the proton temperature anisotropy T ⊥ /T || in bursty bulk flows (BBFs), as observed by THEMIS Mission. For a set of 10 selected events, during which at least three spacecraft are aligned in the same flow, we can sample the plasma parameters along the Earth's magnetotail. The temperature anisotropy in the quiescent tail is negligible. However, as soon as the BBF passes over the spacecraft a strong anisotropy is measured. We analyze T ⊥ /T || as a function of parallel plasma beta-β || (=nkT || /(B 2 /2μ 0)) for the different THEMIS satellites and compare the spread of the data points with various instability thresholds over ion scales that can reduce the temperature anisotropy: for T ⊥ /T || <1 the parallel and oblique firehose; for T ⊥ /T || >1 the proton cyclotron and mirror mode. It is shown that the anisotropy reduces whilst the BBF is moving Earthward, and the strongest fluctuations are enhanced along the instability thresholds, indicating that these instabilities reduce the proton temperature anisotropy.
During the last several years significant progress has been made in understanding MHD turbulence ... more During the last several years significant progress has been made in understanding MHD turbulence in the Earth's plasma sheet. Due to the statistically transitory properties of fluctuations, finite size and boundary effects, however, issues of fundamental importance remain unresolved. Here we concentrate on such intrinsic features of plasma sheet turbulence as its origin and dynamical nature. In particular, we investigate bursty bulk flow driven multi-scale transfer of energy towards the dissipation scale, and provide evidence for the presence of non-linear interactions. We show that, in contrast with previous results, Alfvénic fluctuations together with 2D eddy interactions may appear as important constituents of turbulence in the plasma sheet.
Recent studies provide evidence for the multi-scale nature of magnetic turbulence in the plasma s... more Recent studies provide evidence for the multi-scale nature of magnetic turbulence in the plasma sheet. Wavelet methods represent modern time series analysis techniques suitable for the description of statistical characteristics of multi-scale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field high-resolution (67 Hz) measurements are studied during an interval in which the spacecraft are in the plasma sheet. As Cluster passes through different plasma regions, physical processes exhibit non-steady properties on magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence, the implementation of wavelet-based techniques becomes complicated due to the statistically transitory properties of magnetic fluctuations and finite size effects. Using a supervised multi-scale technique which allows existence test of moments, the robustness of higher-order statistics is investigated. On this basis the properties of magnetic turbulence are investigated for changing thickness of the plasma sheet.
On the basis of ARTEMIS two-probe mission magnetic reconnection (MR) outflow associated magnetic ... more On the basis of ARTEMIS two-probe mission magnetic reconnection (MR) outflow associated magnetic fluctuations and turbulence are analyzed on 19 February 2011. In the deep-tail, at distances between X = 45 − 51R E , evidence for reconnection associated plasma sheet thinning was found, accompanied by heating of the plasma sheet. Correlated flow and field reversals and the large-scale Hall-effect signatures indicated the presence of the reconnection X-line. Within fast reconnection plasma outflows, magnetic fluctuations exhibit the same spectral scaling features and kinked spectra as magnetic fluctuations in the solar wind or in various parts of geospace. It was shown that the proton scale magnetic fluctuations are constrained by oblique firehose, proton cyclotron and mirror instability thresholds. For parallel plasma β > 1, where the thresholds converge, perpendicular magnetic fluctuations are enhanced. Magnetic compressibility decreases with the distance to the neutral sheet, however, near the instability thresholds it is comparable to the values obtained in the solar wind.
Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surfa... more Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70 0 , but the angle can further decrease owing to the motion of the tube with regards to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably can not reach 1 AU.
Journal of Geophysical Research: Space Physics, 2009
The one‐point probability density functions (PDFs) obtained from the Wind spacecraft observations... more The one‐point probability density functions (PDFs) obtained from the Wind spacecraft observations of the magnitude of total magnetic field (B) and the solar wind quasi‐invariant (QI) are investigated at 1 AU during the years 1995 and 1998. It is known from previous studies that the distributions follow in a rather good approximation a lognormal distribution. This indicates that the underlying random multiplicative processes are skewed, the PDFs are nonsymmetric. The concept of kappa distributions generating PDF tails closer to the observed values is introduced. The skewness, characteristic for the multiplicative processes in the solar wind, is treated on the basis of log‐kappa distributions, introduced here for the first time. Normal and lognormal distributions are related in a similar way to each other as the kappa and log‐kappa distributions, although the statistics is based on extensive physics in the former and nonextensive physics in the latter cases. We show that log‐kappa PDF...
Journal of Geophysical Research: Space Physics, 2011
Long-term observations of proton cyclotron waves in the upstream region of Venus raise the questi... more Long-term observations of proton cyclotron waves in the upstream region of Venus raise the question of under which general solar wind conditions these waves are generated and maintained. The waves are characterized by their occurrence at the local proton cyclotron frequency and left-hand polarization, both in the spacecraft frame. Magnetometer data of the Venus Express spacecraft for two Venus years of observations are analyzed before, during, and after the occurrence of these waves. The configuration of the upstream magnetic field and the solar wind velocity is investigated, to study if the waves are generated from a ring distribution of pickup ions in velocity space or from a parallel pickup ion beam, i.e., for quasi-parallel conditions of solar wind velocity and magnetic field when the solar wind motional electric field is weak. It is found that stable and mainly quasi-parallel magnetic field conditions for up to ∼20 min prior to wave observation are present, enabling sufficient ion pickup and wave growth to obtain observable waves in the magnetometer data. Persistent waves occur mainly under quasi-parallel conditions. This is in agreement with linear theory, which predicts efficient wave growth for instabilities driven by field-aligned planetary ion beams, already for low pickup ion density. The occurrence of highly coherent waves at 4 R V upstream toward the Sun implies that planetary neutral hydrogen is initially picked up at least 5 R V toward the Sun from a sufficiently dense Venus hydrogen exosphere.
Journal of Geophysical Research: Space Physics, 2010
The magnetic field fluctuations near Venus are investigated in the frequency range 0.03-0.3 Hz on... more The magnetic field fluctuations near Venus are investigated in the frequency range 0.03-0.3 Hz on the basis of the measurements observed by Venus Express from April 2006 to December 2008. The data are sorted by the angle between interplanetary magnetic field (IMF) and solar wind flow. The spatial distributions of fluctuation properties under the different IMF orientations are presented and a comparative study is performed. In the mantle and tail regions, the magnetic field is fairly quiet and the fluctuations are almost linearly polarized. There are two possible sources for the fluctuations in the magnetosheath: convection from the upstream foreshock and local generation. When the IMF is nearly perpendicular to the solar wind flow, the fluctuations in the magnetosheath are mainly generated locally by an ion cyclotron instability due to planetary ion pickup. The wave intensity is relatively low and the transverse component is dominant. The waves are lefthanded, elliptically polarized, and propagating parallel to the mean magnetic field. When the IMF is nearly aligned to the solar wind flow, foreshock waves are convected into the magnetosheath. The fluctuations in the magnetosheath become more intensive. Their polarization properties are very mixed and there is no clear tendency. It indicates that the waves convected from the foreshock may be the mixture of multiple wave types and incoherent noise.
Journal of Geophysical Research: Space Physics, 2004
Small‐scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investi... more Small‐scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investigated by means of a wavelet estimator suitable for detecting distinct scaling characteristics even in noisy measurements. The spectral estimators used for this purpose are affected by a frequency‐dependent bias. The variances of the wavelet coefficients, however, match the power‐law shaped spectra, which makes the wavelet estimator essentially unbiased. These scaling characteristics of the magnetic field data appear to be essentially nonsteady and intermittent. The scaling properties of bursty bulk flow (BBF) and non‐BBF associated magnetic fluctuations are analyzed with the aim of understanding processes of energy transfer between scales. Small‐scale (∼0.08–0.3 s) magnetic fluctuations having the same scaling index α ∼ 2.6 as the large‐scale (∼0.7–5 s) magnetic fluctuations occur during BBF‐associated periods. During non‐BBF associated periods the energy transfer to small scales is abse...
Venus Express interplanetary magnetic field measurements have been examined for magnetic ''holes,... more Venus Express interplanetary magnetic field measurements have been examined for magnetic ''holes,'' accompanied by magnetic field directional changes. We examine both the thickness of the current sheet and the depth of the magnetic field depression. We find the thickness of the current sheet is not correlated with the depth of the field depression. The depth of the magnetic holes is related to directional angle change. Since total pressure should balance across these discontinuities, there must be enhanced plasma pressure within the magnetic holes. The dependence of the depth of the hole (i.e., size of the plasma pressure enhancement) on the directional changes suggests that the heating of the plasma associated with the hole formation may be provided by annihilation of the magnetic energy in the current sheet, via slow reconnection.
Using multi-satellite Cluster magnetic field data, we statistically examined how often an atypica... more Using multi-satellite Cluster magnetic field data, we statistically examined how often an atypical, i.e., non-Harris type, current sheet is formed. We found that a cross-tail current sheet with an off-equatorial current density maximum, indicating bifurcation, is frequently formed in thin current sheets with a half thickness of $1500 km. The occurrence of off-equatorial current density maxima is correlated with the occurrence of fast plasma flows, namely if fast plasma flows occur nearby, one is more likely to observe off-equatorial current density maxima. We also found that many of the center-peaked current sheets exhibit a feature different from a normal Harris sheet: namely a very intense current concentrated near the equatorial plane, embedded in a broader current sheet with lower current density.
Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes ... more Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes results in Kelvin-Helmholtz instability, which might contribute to solar wind turbulence. While the axial magnetic field stabilizes the instability, a small twist in the magnetic field may allow sub-Alfvénic motions to be unstable. Aims. We aim to study the Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind with different configurations of the external magnetic field. Methods. We use magnetohydrodynamic equations in cylindrical geometry and derive the dispersion equations governing the dynamics of twisted magnetic flux tubes moving along its axis in the cases of untwisted and twisted external fields. Then, we solve the dispersion equations analytically and numerically and find thresholds for Kelvin-Helmholtz instability in both cases of the external field. Results. Both analytical and numerical solutions show that the Kelvin-Helmholtz instability is suppressed in the twisted tube by the external axial magnetic field for sub-Alfvénic motions. However, even a small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any sub-Alfvénic motion. The unstable harmonics correspond to vortices with high azimuthal mode numbers that are carried by the flow. Conclusions. Twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to the main solar wind stream, then the Kelvin-Helmholtz vortices may significantly contribute to the solar wind turbulence.
Formation of a stationary ring current system localized near the equatorial plane of a quasi-dipo... more Formation of a stationary ring current system localized near the equatorial plane of a quasi-dipolar magnetic configuration is considered. As a driver for this current system, appears the expanding radial flow of partially ionized plasma. Obtained analytical solutions, numerical simulation and laboratory experiments confirm a possibility of similar effect in the inner magnetosphere of a Hot Jupiter in the presence of an expanding flow of the escaping upper atmosphere of the planet.
A re-evaluation of the reconnection event reported by Volwerk et al. (2009) shows that the origin... more A re-evaluation of the reconnection event reported by Volwerk et al. (2009) shows that the original interpretation of the magnetic field data as quadrupolar Hall-current signatures around a reconnection site was mistaken. It could be interpreted as the signature of reconnection in the presence of a guide field. The path of VEX through the active region in Venus's magnetotail is re-evaluated and the strongly energized ions associated to this event are now in agreement with the magnetic field data.
Petschek-type time-dependent reconnection (TDR) and quasi-stationary reconnection (QSR) models ar... more Petschek-type time-dependent reconnection (TDR) and quasi-stationary reconnection (QSR) models are considered to understand reconnection outflow structures and the generation of local turbulence in the solar wind. Comparing TDR/QSR model predictions of the outflow structures with actual measurements shows that both models can explain the data equally well. It is demonstrated that the outflows can often generate more or less spatially extended turbulent boundary layers. The structure of a unique extended reconnection outflow is investigated in detail. The analysis of spectral scalings and spectral break locations shows that reconnection can change the local field and plasma conditions which may support different local turbulent dissipation mechanisms at their characteristic wavenumbers.
Proceedings of the International Astronomical Union, 2004
In this paper we compare the intermittence properties of magnetic fluctuations (non-Gaussian shap... more In this paper we compare the intermittence properties of magnetic fluctuations (non-Gaussian shape of probability density functions) observed in the solar wind (ACE) with the simultaneous occurrence of intermittence in the Earth's plasma sheet (GEOTAIL). Intervals with different level of magnetic turbulence are investigated separately.
Three consecutive turbulent magnetic dipolarizations accompanied by auroral brightenings near the... more Three consecutive turbulent magnetic dipolarizations accompanied by auroral brightenings near the equatorward boundary of wide auroral oval were observed with fortuitous spacecraft constellation on September 26, 2005. All were associated with strong near-Earth reconnection pulses (at r ≤ 14Re with Cluster probed the tailward reconnection outflow region) with separatrix mapped to ∼ 64 • CGLat in the ionosphere where a narrow energy-dispersed ion injection was observed. Onset of magnetic reconnection was nearly simultaneous or lead as compared to the turbulent dipolarization and energetic particle injection onsets. The reconnection tailward outflow contained intense turbulence with the properties similar to that in the turbulent dipolarization regions and with intensity correlating with the outflow amplitude. We conclude that the reconnection process and the growth of strong turbulence in the near tail are strongly coupled together, at least in near-Earth reconnection events, and that near-Earth location of the reconnection site may be more frequent phenomenon than typically thought. In that case it assumed to be possible due to enhanced SW flow pressure which kept the magnetic configuration very stretched in the absence of strong energy loading into the magnetosphere; the ground magnetic perturbations ranged between 50 and 300nT in these intense reconnection events.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this p... more The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.
Journal of Geophysical Research-Space Physics, 2013
We study the development of the proton temperature anisotropy T ⊥ /T || in bursty bulk flows (BBF... more We study the development of the proton temperature anisotropy T ⊥ /T || in bursty bulk flows (BBFs), as observed by THEMIS Mission. For a set of 10 selected events, during which at least three spacecraft are aligned in the same flow, we can sample the plasma parameters along the Earth's magnetotail. The temperature anisotropy in the quiescent tail is negligible. However, as soon as the BBF passes over the spacecraft a strong anisotropy is measured. We analyze T ⊥ /T || as a function of parallel plasma beta-β || (=nkT || /(B 2 /2μ 0)) for the different THEMIS satellites and compare the spread of the data points with various instability thresholds over ion scales that can reduce the temperature anisotropy: for T ⊥ /T || <1 the parallel and oblique firehose; for T ⊥ /T || >1 the proton cyclotron and mirror mode. It is shown that the anisotropy reduces whilst the BBF is moving Earthward, and the strongest fluctuations are enhanced along the instability thresholds, indicating that these instabilities reduce the proton temperature anisotropy.
During the last several years significant progress has been made in understanding MHD turbulence ... more During the last several years significant progress has been made in understanding MHD turbulence in the Earth's plasma sheet. Due to the statistically transitory properties of fluctuations, finite size and boundary effects, however, issues of fundamental importance remain unresolved. Here we concentrate on such intrinsic features of plasma sheet turbulence as its origin and dynamical nature. In particular, we investigate bursty bulk flow driven multi-scale transfer of energy towards the dissipation scale, and provide evidence for the presence of non-linear interactions. We show that, in contrast with previous results, Alfvénic fluctuations together with 2D eddy interactions may appear as important constituents of turbulence in the plasma sheet.
Recent studies provide evidence for the multi-scale nature of magnetic turbulence in the plasma s... more Recent studies provide evidence for the multi-scale nature of magnetic turbulence in the plasma sheet. Wavelet methods represent modern time series analysis techniques suitable for the description of statistical characteristics of multi-scale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field high-resolution (67 Hz) measurements are studied during an interval in which the spacecraft are in the plasma sheet. As Cluster passes through different plasma regions, physical processes exhibit non-steady properties on magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence, the implementation of wavelet-based techniques becomes complicated due to the statistically transitory properties of magnetic fluctuations and finite size effects. Using a supervised multi-scale technique which allows existence test of moments, the robustness of higher-order statistics is investigated. On this basis the properties of magnetic turbulence are investigated for changing thickness of the plasma sheet.
On the basis of ARTEMIS two-probe mission magnetic reconnection (MR) outflow associated magnetic ... more On the basis of ARTEMIS two-probe mission magnetic reconnection (MR) outflow associated magnetic fluctuations and turbulence are analyzed on 19 February 2011. In the deep-tail, at distances between X = 45 − 51R E , evidence for reconnection associated plasma sheet thinning was found, accompanied by heating of the plasma sheet. Correlated flow and field reversals and the large-scale Hall-effect signatures indicated the presence of the reconnection X-line. Within fast reconnection plasma outflows, magnetic fluctuations exhibit the same spectral scaling features and kinked spectra as magnetic fluctuations in the solar wind or in various parts of geospace. It was shown that the proton scale magnetic fluctuations are constrained by oblique firehose, proton cyclotron and mirror instability thresholds. For parallel plasma β > 1, where the thresholds converge, perpendicular magnetic fluctuations are enhanced. Magnetic compressibility decreases with the distance to the neutral sheet, however, near the instability thresholds it is comparable to the values obtained in the solar wind.
Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surfa... more Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70 0 , but the angle can further decrease owing to the motion of the tube with regards to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably can not reach 1 AU.
Journal of Geophysical Research: Space Physics, 2009
The one‐point probability density functions (PDFs) obtained from the Wind spacecraft observations... more The one‐point probability density functions (PDFs) obtained from the Wind spacecraft observations of the magnitude of total magnetic field (B) and the solar wind quasi‐invariant (QI) are investigated at 1 AU during the years 1995 and 1998. It is known from previous studies that the distributions follow in a rather good approximation a lognormal distribution. This indicates that the underlying random multiplicative processes are skewed, the PDFs are nonsymmetric. The concept of kappa distributions generating PDF tails closer to the observed values is introduced. The skewness, characteristic for the multiplicative processes in the solar wind, is treated on the basis of log‐kappa distributions, introduced here for the first time. Normal and lognormal distributions are related in a similar way to each other as the kappa and log‐kappa distributions, although the statistics is based on extensive physics in the former and nonextensive physics in the latter cases. We show that log‐kappa PDF...
Journal of Geophysical Research: Space Physics, 2011
Long-term observations of proton cyclotron waves in the upstream region of Venus raise the questi... more Long-term observations of proton cyclotron waves in the upstream region of Venus raise the question of under which general solar wind conditions these waves are generated and maintained. The waves are characterized by their occurrence at the local proton cyclotron frequency and left-hand polarization, both in the spacecraft frame. Magnetometer data of the Venus Express spacecraft for two Venus years of observations are analyzed before, during, and after the occurrence of these waves. The configuration of the upstream magnetic field and the solar wind velocity is investigated, to study if the waves are generated from a ring distribution of pickup ions in velocity space or from a parallel pickup ion beam, i.e., for quasi-parallel conditions of solar wind velocity and magnetic field when the solar wind motional electric field is weak. It is found that stable and mainly quasi-parallel magnetic field conditions for up to ∼20 min prior to wave observation are present, enabling sufficient ion pickup and wave growth to obtain observable waves in the magnetometer data. Persistent waves occur mainly under quasi-parallel conditions. This is in agreement with linear theory, which predicts efficient wave growth for instabilities driven by field-aligned planetary ion beams, already for low pickup ion density. The occurrence of highly coherent waves at 4 R V upstream toward the Sun implies that planetary neutral hydrogen is initially picked up at least 5 R V toward the Sun from a sufficiently dense Venus hydrogen exosphere.
Journal of Geophysical Research: Space Physics, 2010
The magnetic field fluctuations near Venus are investigated in the frequency range 0.03-0.3 Hz on... more The magnetic field fluctuations near Venus are investigated in the frequency range 0.03-0.3 Hz on the basis of the measurements observed by Venus Express from April 2006 to December 2008. The data are sorted by the angle between interplanetary magnetic field (IMF) and solar wind flow. The spatial distributions of fluctuation properties under the different IMF orientations are presented and a comparative study is performed. In the mantle and tail regions, the magnetic field is fairly quiet and the fluctuations are almost linearly polarized. There are two possible sources for the fluctuations in the magnetosheath: convection from the upstream foreshock and local generation. When the IMF is nearly perpendicular to the solar wind flow, the fluctuations in the magnetosheath are mainly generated locally by an ion cyclotron instability due to planetary ion pickup. The wave intensity is relatively low and the transverse component is dominant. The waves are lefthanded, elliptically polarized, and propagating parallel to the mean magnetic field. When the IMF is nearly aligned to the solar wind flow, foreshock waves are convected into the magnetosheath. The fluctuations in the magnetosheath become more intensive. Their polarization properties are very mixed and there is no clear tendency. It indicates that the waves convected from the foreshock may be the mixture of multiple wave types and incoherent noise.
Journal of Geophysical Research: Space Physics, 2004
Small‐scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investi... more Small‐scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investigated by means of a wavelet estimator suitable for detecting distinct scaling characteristics even in noisy measurements. The spectral estimators used for this purpose are affected by a frequency‐dependent bias. The variances of the wavelet coefficients, however, match the power‐law shaped spectra, which makes the wavelet estimator essentially unbiased. These scaling characteristics of the magnetic field data appear to be essentially nonsteady and intermittent. The scaling properties of bursty bulk flow (BBF) and non‐BBF associated magnetic fluctuations are analyzed with the aim of understanding processes of energy transfer between scales. Small‐scale (∼0.08–0.3 s) magnetic fluctuations having the same scaling index α ∼ 2.6 as the large‐scale (∼0.7–5 s) magnetic fluctuations occur during BBF‐associated periods. During non‐BBF associated periods the energy transfer to small scales is abse...
Venus Express interplanetary magnetic field measurements have been examined for magnetic ''holes,... more Venus Express interplanetary magnetic field measurements have been examined for magnetic ''holes,'' accompanied by magnetic field directional changes. We examine both the thickness of the current sheet and the depth of the magnetic field depression. We find the thickness of the current sheet is not correlated with the depth of the field depression. The depth of the magnetic holes is related to directional angle change. Since total pressure should balance across these discontinuities, there must be enhanced plasma pressure within the magnetic holes. The dependence of the depth of the hole (i.e., size of the plasma pressure enhancement) on the directional changes suggests that the heating of the plasma associated with the hole formation may be provided by annihilation of the magnetic energy in the current sheet, via slow reconnection.
Using multi-satellite Cluster magnetic field data, we statistically examined how often an atypica... more Using multi-satellite Cluster magnetic field data, we statistically examined how often an atypical, i.e., non-Harris type, current sheet is formed. We found that a cross-tail current sheet with an off-equatorial current density maximum, indicating bifurcation, is frequently formed in thin current sheets with a half thickness of $1500 km. The occurrence of off-equatorial current density maxima is correlated with the occurrence of fast plasma flows, namely if fast plasma flows occur nearby, one is more likely to observe off-equatorial current density maxima. We also found that many of the center-peaked current sheets exhibit a feature different from a normal Harris sheet: namely a very intense current concentrated near the equatorial plane, embedded in a broader current sheet with lower current density.
Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes ... more Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes results in Kelvin-Helmholtz instability, which might contribute to solar wind turbulence. While the axial magnetic field stabilizes the instability, a small twist in the magnetic field may allow sub-Alfvénic motions to be unstable. Aims. We aim to study the Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind with different configurations of the external magnetic field. Methods. We use magnetohydrodynamic equations in cylindrical geometry and derive the dispersion equations governing the dynamics of twisted magnetic flux tubes moving along its axis in the cases of untwisted and twisted external fields. Then, we solve the dispersion equations analytically and numerically and find thresholds for Kelvin-Helmholtz instability in both cases of the external field. Results. Both analytical and numerical solutions show that the Kelvin-Helmholtz instability is suppressed in the twisted tube by the external axial magnetic field for sub-Alfvénic motions. However, even a small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any sub-Alfvénic motion. The unstable harmonics correspond to vortices with high azimuthal mode numbers that are carried by the flow. Conclusions. Twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to the main solar wind stream, then the Kelvin-Helmholtz vortices may significantly contribute to the solar wind turbulence.
Formation of a stationary ring current system localized near the equatorial plane of a quasi-dipo... more Formation of a stationary ring current system localized near the equatorial plane of a quasi-dipolar magnetic configuration is considered. As a driver for this current system, appears the expanding radial flow of partially ionized plasma. Obtained analytical solutions, numerical simulation and laboratory experiments confirm a possibility of similar effect in the inner magnetosphere of a Hot Jupiter in the presence of an expanding flow of the escaping upper atmosphere of the planet.
A re-evaluation of the reconnection event reported by Volwerk et al. (2009) shows that the origin... more A re-evaluation of the reconnection event reported by Volwerk et al. (2009) shows that the original interpretation of the magnetic field data as quadrupolar Hall-current signatures around a reconnection site was mistaken. It could be interpreted as the signature of reconnection in the presence of a guide field. The path of VEX through the active region in Venus's magnetotail is re-evaluated and the strongly energized ions associated to this event are now in agreement with the magnetic field data.
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Papers by Zoltan Voros